Improvements to namespace support

This issue is just to track the implementation plan laid out at the bottom of https://github.com/dtolnay/cxx/pull/298#issuecomment-703817493.

• Emit all mentions of a type in generated C++ code (in function arguments and so forth) as a fully qualified path ::name::space::of::Type, even if it currently always happens to be within the same module. #370

• Store a distinct Namespace on each FFI item rather than having a single global namespace for the whole code generation. #370

• Implement #[namespace = ...] attribute to determine namespace of individual items. #370 #380

• Implement hierarchical handling of namespace which prioritizes item-level attribute > extern block > module. #444

• Emit ExternType impls for shared structs. #341

• Safe ExternType derive for extern Rust items. extern "Rust" { #[derive(ExternType)] type TheType; } 16e2620fe51032d789770b489e27a9c6c27719ef

• Permit use inside of cxx::bridge.